A detailed design of a picosecond laser oscillator is made by using optical resonance theory and semiconductor saturable absorber mirror continuous wave mode-locked technology. Mode parameters in the optical resonance including beam sizes on the laser crystal and mode locker are calculated. By theoretical calculations, 3.7W output power is obtained at a pump power of 11W and the optical to optical efficiency is 34% in the designed model of picosecond laser. Based on the detailed design, an experiment is proceeded and a picosecond laser oscillator of about 3.5W output power with 10.6W pump power is fabricated. The optical to optical efficiency of the laser is 33%, the pulse duration is about 20ps, and the repetition rate is about 80.3MHz. The oscillator presents long-term stability in the experiment.